RPM Support

Steps to reproduce. I recommend sticking a sleep or debugger before this line: https://github.com/pulp/pulp_rpm/blob/d313a990965ccfa3c15bc1443ae77cfad486ad30/pulp_rpm/app/tasks/publishing.py#L228

1. Create a publication. Make sure this task is running during during step 2.
2. List publications with the bindings (or simply access the /pulp/api/v3/publications/rpm/rpm/ to confirm that the publication is returned with empty string for metadata_checksum_type)

Error:

Traceback (most recent call last):
  File "./listpub.py", line 15, in <module>
    PublicationsRpmApi(rpm_client).list().results
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api/publications_rpm_api.py", line 289, in list
    return self.list_with_http_info(**kwargs)  # noqa: E501
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api/publications_rpm_api.py", line 401, in list_with_http_info
    collection_formats=collection_formats)
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api_client.py", line 353, in call_api
    _preload_content, _request_timeout, _host)
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api_client.py", line 192, in __call_api
    return_data = self.deserialize(response_data, response_type)
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api_client.py", line 264, in deserialize
    return self.__deserialize(data, response_type)
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api_client.py", line 303, in __deserialize
    return self.__deserialize_model(data, klass)
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api_client.py", line 639, in __deserialize_model
    kwargs[attr] = self.__deserialize(value, attr_type)
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api_client.py", line 281, in __deserialize
    for sub_data in data]
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api_client.py", line 281, in <listcomp>
    for sub_data in data]
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api_client.py", line 303, in __deserialize
    return self.__deserialize_model(data, klass)
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/api_client.py", line 641, in __deserialize_model
    instance = klass(**kwargs)
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/models/rpm_rpm_publication.py", line 76, in __init__
    self.metadata_checksum_type = metadata_checksum_type
  File "/opt/utils/venv/pulp/3.7.3/lib64/python3.7/site-packages/pulpcore/client/pulp_rpm/models/rpm_rpm_publication.py", line 192, in metadata_checksum_type
    .format(metadata_checksum_type, allowed_values)
ValueError: Invalid value for `metadata_checksum_type` (), must be one of ['unknown', 'md5', 'sha1', 'sha224', 'sha256', 'sha384', 'sha512']